su - zimbra
zmprov mcf zimbraLogRawLifetime Xd (31d by default)
zmprov mcf zimbraLogSummaryLifetime Xd (730d by default)
If you're curious how this process works examine /opt/zimbra/libexec/zmlogprocess (which is fired off every 10 min from crontab against the zimbra_logger DB). It's used in things like zmmstgrace.